How can I capture STDOUT to a string?

Question

puts "hi"
puts "bye"

I want to store the STDOUT of the code so far (in this case hi \nbye into a variable say 'result' and print it )

puts result

The reason I am doing this is I have integrate an R code into my Ruby code, output of which is given to the STDOUT as the R code runs , but the ouput cannot be accessed inside the code to do some evaluations. Sorry if this is confusing. So the "puts result" line should give me hi and bye.

Answer 1

A handy function for capturing stdout into a string...

The following method is a handy general purpose tool to capture stdout and return it as a string. (I use this frequently in unit tests where I want to verify something printed to stdout.) Note especially the use of the ensure clause to restore $stdout (and avoid astonishment):

def with_captured_stdout
  original_stdout = $stdout  # capture previous value of $stdout
  $stdout = StringIO.new     # assign a string buffer to $stdout
  yield                      # perform the body of the user code
  $stdout.string             # return the contents of the string buffer
ensure
  $stdout = original_stdout  # restore $stdout to its previous value
end

So, for example:

>> str = with_captured_stdout { puts "hi"; puts "bye"}
=> "hi\nbye\n"
>> print str
hi
bye
=> nil

Answer 2

Redirect Standard Output to a StringIO Object

You can certainly redirect standard output to a variable. For example:

# Set up standard output as a StringIO object.
foo = StringIO.new
$stdout = foo

# Send some text to $stdout.
puts 'hi'
puts 'bye'

# Access the data written to standard output.
$stdout.string
# => "hi\nbye\n"

# Send your captured output to the original output stream.
STDOUT.puts $stdout.string

In practice, this is probably not a great idea, but at least now you know it's possible.

Answer 3

If activesupport is available in your project you may do the following:

output = capture(:stdout) do
  run_arbitrary_code
end

More info about Kernel.capture can be found here

Answer 4

You can do this by making a call to your R script inside backticks, like this:

result = `./run-your-script`
puts result  # will contain STDOUT from run-your-script

For more information on running subprocesses in Ruby, check out this Stack Overflow question.

Answer 5

Capture stdout (or stderr) for both Ruby code and subprocesses

# capture_stream(stream) { block } -> String
#
# Captures output on +stream+ for both Ruby code and subprocesses
# 
# === Example
#
#    capture_stream($stdout) { puts 1; system("echo 2") }
# 
# produces
# 
#    "1\n2\n"
#
def capture_stream(stream)
  raise ArgumentError, 'missing block' unless block_given?
  orig_stream = stream.dup
  IO.pipe do |r, w|
    # system call dup2() replaces the file descriptor 
    stream.reopen(w) 
    # there must be only one write end of the pipe;
    # otherwise the read end does not get an EOF 
    # by the final `reopen`
    w.close 
    t = Thread.new { r.read }
    begin
      yield
    ensure
      stream.reopen orig_stream # restore file descriptor 
    end
    t.value # join and get the result of the thread
  end
end

I got inspiration from Zhon.

Answer 6

For most practical purposes you can put anything into $stdout that responds to write, flush, sync, sync= and tty?.

In this example I use a modified Queue from the stdlib.

class Captor < Queue

  alias_method :write, :push

  def method_missing(meth, *args)
    false
  end

  def respond_to_missing?(*args)
    true
  end
end

stream = Captor.new
orig_stdout = $stdout
$stdout = stream

puts_thread = Thread.new do
  loop do
    puts Time.now
    sleep 0.5
  end
end

5.times do
  STDOUT.print ">> #{stream.shift}"
end

puts_thread.kill
$stdout = orig_stdout

You need something like this if you want to actively act on the data and not just look at it after the task has finished. Using StringIO or a file will have be problematic with multiple threads trying to sync reads and writes simultaneously.

Answer 7

Minitest versions:

---

assert_output if you need to ensure if some output is generated:

assert_output "Registrars processed: 1\n" do
  puts 'Registrars processed: 1'
end

assert_output

---

or use capture_io if you really need to capture it:

out, err = capture_io do
  puts "Some info"
  warn "You did a bad thing"
end

assert_match %r%info%, out
assert_match %r%bad%, err

capture_io

Minitest itself is available in any Ruby version starting from 1.9.3

Answer 8

For RinRuby, please know that R has capture.output:

R.eval <<EOF
captured <- capture.output( ... )
EOF

puts R.captured 

Answer 9

Credit to @girasquid's answer. I modified it to a single file version:

def capture_output(string)
  `echo #{string.inspect}`.chomp
end

# example usage
response_body = "https:\\x2F\\x2Faccounts.google.com\\x2Faccounts"
puts response_body #=> https:\x2F\x2Faccounts.google.com\x2Faccounts
capture_output(response_body) #=> https://accounts.google.com/accounts